When you buy a UX designer, what are you actually buying?

Once, to do web design you had to be a T-shaped person. This is defined as a person who knows a little bit about many things and a lot about one thing. Imagine a programmer who also understands a bit about business models and some interface design. But as our product complexity grows, we need P and M shaped people–people with multiple deep specialties. To design great user experiences, you need to specialize in a combination of brand management, interaction design, human-computer factors and business model design. Or you could be part of a team. The term UX was welcomed because we finally had an umbrella of related practices.

Of course, we don’t all belong to the same version of that umbrella. We all bring different focuses under the umbrella, different experiences, mindsets, and practices. While we can all learn from each other, we can’t always be each other.

But trouble started when our clients didn’t realize it was an umbrella, and thought it was a person. And they tried to hire them.

- User Experience Go Away, by Dave Malouf, emphasis mine.

FWIW, Malouf's definition is pretty close to my own, in terms of what's needed. =)